Since inkjet image forming methods have such advantages as low noise and low running costs over other image forming methods, the inkjet image forming methods become widely used recently. In addition, inkjet printers capable of forming full color images on plain paper are actively marketed recently. However, it is difficult for such inkjet inks to fulfill all the requirements such that images having a good combination of color reproducibility, abrasion resistance, durability, light resistance, and drying property can be produced without causing problems such as feathering (blurring), color bleeding (i.e., blurring on the boundary portion of color images), curl of copies, and nozzle clogging in that ink ejection nozzles are clogged with the inks; and duplex copies can be produced without causing any problem such as penetration of ink into the backside of a recording material. Therefore, inkjet inks fulfilling higher-priority requirements of an inkjet printer are used for the inkjet printer now.
Inkjet inks typically include water as a main component while including a colorant, and a wetting agent such as glycerin, which is added to prevent occurrence of the nozzle clogging problem. Dyes or pigments are used for the colorant. Dyes are superior to pigments in coloring property, but are inferior to pigments in resistance to light, gasses and water. In other words, pigments are superior to dyes in resistance to light, gasses and water, and are inferior to dyes in coloring property.
Specifically, dye-based inkjet inks tend to cause a blurring problem in that images formed on plain paper are blurred, particularly, at boundary portions of a black ink image and a color ink image (i.e., this phenomenon is hereinafter referred to as color bleeding). In attempting to solve the color bleeding problem, there is a proposal to use a combination of a black ink and color inks having higher permeability to paper than the black ink, so that the black ink in a black image does not penetrate into images of the color inks. In this proposal, the permeability is controlled by adjusting the added amount of a surfactant included in the inks. However, the color bleeding property of such a dye-based ink is still inferior to that of a pigment-based ink. In addition, there is a proposal to use inks having a relatively high viscosity to lower the permeability of the inks. However, in order to stably eject inks from nozzles, the viscosity of inks has an upper limit, and therefore good effect cannot be produced by the technique.
With respect to inkjet recording methods, serial recording methods in which an image is formed on a recording material while moving a recording head in a main scanning direction (i.e., in the width direction of the recording material) have been conventionally used. When images are recorded on A-4 paper sheets at a pixel density of 600×300 dpi by using a printer using a serial recording method, the copying speed is 30 copies per minute at most.
However, recently a line printer having a line recording head having the same length as the maximum width of recording materials is developed. By using such a line printer, more than 200 copies can be produced per minute when images are recorded on A-4 paper sheets at a pixel density of 600×300 dpi. Thus, high speed recording has also been pursued in inkjet recording.
When high speed inkjet recording is performed, a curling problem in that images are recorded on a plain paper using aqueous inks, the resultant copy has large curl is easily caused. Since a seriously curled copy is easily jammed in a sheet feeding passage of a printer, it is difficult for the printer to perform high speed inkjet recording (hereinafter this problem is referred to as a jamming problem). The reason why a copy having an image formed by an aqueous ink is curled is considered to be as follows. Specifically, water included in ink images formed on a paper sheet serving as a recording material penetrates into cellulose fibers in the paper sheet while cutting the hydrogen bonds between the cellulose fibers of the paper sheet, thereby swelling (i.e., extending) the surface of the paper sheet, on which the image is formed. In this regard, since the backside of the paper sheet is not swelled (i.e., not extended), the paper sheet is curled toward the backside.
In inkjet recording, high speed recording can be easily performed if the ink droplets ejected by nozzles have relatively large size. In this case, the amount of inks adhered to a recording material per a unit area increases, and therefore the resultant copy has relatively large curl. In addition, sharpness and half tone property of the recorded images deteriorate. In contrast, when the ink droplets ejected by nozzles have relatively small size, occurrence of the above-mentioned problems can be prevented, but the recording time seriously increases, resulting in deterioration of copy productivity to an extent such that the recording method cannot be practically used.
In attempting to solve the curling problem, a technique such that a print having an image thereon is heated to evaporate water included therein is also proposed. However, since a large amount of energy is needed for heating the print, the printer loses one of the advantages, i.e., energy saving. In addition, the inkjet printer is required to have a heating space, the printer loses another advantage, space saving.
Although curl is formed on a paper sheet just after recording images thereon, the degree of curl is reduced with time because water penetrating into the paper sheet evaporates. However, even when water included in the ink images completely evaporates, the curled copy does not return to the original form of the paper sheet (i.e., a flat form). Therefore, it is difficult to practically use a recording method forming such a curled copy.
In contrast, oil-based inks do not cause the curling problem because of including no water, and therefor oil-based inks are considered to be suitable for high speed recording. However, oil-based inkjet inks typically have a low viscosity, and therefore the inks penetrate into recording papers at a high speed, resulting in formation of low density images. In addition, such inks penetrate into the backside of paper sheets (hereinafter referred to as a ink penetration problem), duplex copies cannot be formed. Further, since recorded images are seriously blurred, character images having good image qualities cannot be formed. Therefore, oil-based inkjet inks are used only for limited applications now.
For these reasons, the inventors recognized that there is a need for an inkjet ink set which can produce high quality images without causing problems such as the curling problem, the ink penetration problem and the bleeding problem mentioned above.